Intrinsically conducting polymers (ICP) are polymers with extended π conjugation along the molecular backbone. Oxidation (p-doping) of the polymer transforms the chains from neutral species to poly(cation)s. Macroscopically the electrical conductivity of these polymers increases by several orders of magnitude upon oxidation. The delocalization of the positive charges (holes) along the conjugated π electron system is responsible for the conductivity of these materials. Because of their extended π conjugation and poly(ionic) nature, most conducting polymers do not melt or dissolve in common solvents, and therefore can not be processed.
Intrinsically conducting polymers have been produced chemically from a solvated mixture of monomer(s) and oxidizers. The polymerization takes place as monomer units are oxidized to form radical cations. These radical species couple with each other with the loss of two protons, forming a colvalent bond. Doping also occurs simultaneously with polymerization via oxidation and results in the formation of positively charged conducting polymer chains. This ionic character causes aggregation of polymer chains, and leads to an insoluble material that precipitates from solution during the oxidative polymerization process. Overall, what results from the oxidative polymerization reaction is a solvent mixture of doped polymer precipitate, spent oxidizing agent, acid, and any unreacted monomer or oxidizing agent. Typically, the intrinsically conducting polymer product is purified by filtration from the above mixture to a “dried” state (the terms dried, dry, drying, etc. herein are used broadly to refer generally to removal of any organic solvent, water and mixtures thereof to form a solid residue or powder) followed by further washing and drying of the filtered product.
The advantage of the oxidative chemical polymerization described above is that the polymer is obtained in its stable doped state. High conductivity or heavy doping can provide the desired electronic properties in a given material, but this very same process leads to a highly ionic character along the polymer chains. Again, this ionic character causes aggregation and precipitation of polymer chains to form an insoluble product.
Poly(heteroaromatic) polymers are a class of ICPs made by oxidative polymerization of heteroaromatic monomers (see below). Many methods for chemically polymerizing heteroaromatic ICPs are known to those skilled in the art. In general, chemical polymerization is carried out by the combination of a chemical oxidizing agent and one or more heteroaromatic monomer(s). The polymerization is usually carried out in water, a polar organic solvent, or a mixture thereof. However, the procedure leads to the formation of the oxidized or doped form of the heteroaromatic ICP. This doped form is positively charged and retains charge-balancing anions. The ionic nature of this material causes it to self-aggregate; the particles formed from this aggregation become insoluble and precipitate out of solution.
For example, McCullough has described what is known as the “ferric chloride” or “FeCl3” method of polymerizing polythiophenes, a type of poly(heteroaromatic) ICP. This method involves the use of FeCl3 as the oxidant combined with one or more thiophene monomers all dissolved and reacted in chloroform [McCullough, R. D., Advanced Materials, 10, (1998) 93-116].
Lefebvre et al. describe a method for producing poly(styrene sulfonate)-doped poly(3,4-ethylenedioxythiophene) or PEDOT (another poly(heteroaromatic) ICP)) using either ferric chloride or ferric nitrate in water or in water/acetonitrile mixtures. The described method resulted in “deep blue powders” that were filtered, washed, and dried to yield an insoluble, intractable powder. They also describe the formation of dispersions of these powders in 20% suspensions of poly(tetrafluoroethylene) in some unidentified solvent; the details of this procedure, including the solvent used are not provided. This suspension was treated with ultrasound and “spread uniformly onto carbon fiber paper” for use as an electrode structure. No characterization was carried out on this film other than electrochemistry [Lefebvre, M. et al., Chemistry of Materials, 11, (1999) 262-268].
The formation of non-poly(heteroaromatic) ICPs as dispersible or otherwise processable solids has been reported. U.S. Pat. No. 5,567,355 (Wessling, B. et al.) reports a process for the formation of intrinsically conducting polyaniline powder in the form of a dispersible dry solid. The preparation process of this patent is specific to the chemistry and electrochemistry of the aniline monomer and polymer. U.S. Pat. No. 4,935,164 (Wessling, B. M. et al.) reports a process for forming blends of ICPs in liquid thermoplastics or solvent dissolved plastics. Examples reported include the use of polyacetylene and polypyrrole as the electrically conducting filler in blends, but poly-para-phenylenes and polyanilines are also reported to be useful. The reported process and blends formed from that process use a dry solid form of the ICP to improve the conductivity of the resulting blend. U.S. Pat. No. 5,254,633 (Han, et al.) reports a process for preparing polyaniline-coated polymer or copolymer particles. They also report the preparation of polymer-particle blends incorporating these polyaniline-coated particles to impart conducting properties to the overall blend.
U.S. Pat. No. 5,498,761 (Wessling, B. et al.) reports a process for the production of ICP films on substrates by coating from a “metastable dispersion”. This patent states that “the concentration of the dispersed conductive polymers in the solvents can be selected within in a very wide range, from almost zero (e.g. 10−5%) to over 5%”. In addition, this patent teaches that gel formation in the ICP compositions is undesirable for use in forming thin films.
Published PCT application WO03018648 (published Mar. 6, 2003) and published U.S. patent application 20030088032 (published May 8, 2003) (Luebben et al.) report block copolymers containing at least one block of a poly(heteroaromatic) polymer and at least two blocks of a non-conjugated polymer. The chemically different blocks of the copolymer are covalently bonded to each other in an alternating fashion through an appropriate linkage group. The poly(heteroaromatic) polymer is an intrinsically conducting polymer (ICP) and when the ICP block or blocks of the block copolymer are in the doped form, the block copolymer is electrically conducting. These applications also provide a method for the preparation of such block copolymers comprising a first step in which a non-conducting block of appropriate molecular weight is modified with one or two linkage groups that undergo oxidative polymerization, and a second step in which the modified non-conducting block is copolymerized with a heteroaromatic monomer under oxidative conditions to form a tri- or a multi-block copolymer. Tri-block copolymers are formed using non-conducting blocks that have one linkage group, while multi-block copolymers are formed using non-conducting blocks that have two linkage groups. Tri-block copolymers have the form BAB (where “A” is the poly(heteroaromatic conducting block, and “B” is a non-conducting polymeric segment), while multi-block copolymers contain a minimum of 4 blocks (e.g. ABAB).
Published applications WO03018648 and US 20030088032 report that copolymers containing blocks of poly(heteroaromatic) ICPs spaced by blocks of non conducting polymers form finely-divided dispersions in organic solvents, and that these dispersions can be used to cast thin films. The block copolymers reported in these applications are easier to process than the corresponding intrinsically conducting homopolymers. While these applications can provide dispersions of poly(heteroaromatic) copolymers which are sufficiently stable to be used in practical commercial applications immediately after their preparation, the present invention provides improved polymerization, purification and dispersion techniques which result in significant improvements in shelf life and long-term stability of such dispersions which further promotes the commercial utility of ICP polymers. Furthermore, dispersions prepared according to this invention have small average number particle size, narrow particle size distribution, and good wetting properties on glass and organic substrates. Thin films prepared from these dispersions are uniform and electrically conducting.
There remains a significant and continuing need in the art for conducting polymers that exhibit improved processability and mechanical and physical properties, and stable forms of solutions or dispersions that have sufficient lifetime (a minimum shelf-life of 3 months is preferred) to allow their use as ready-to-use commercial products.